The oncoprotein Mdm2, which is frequently overexpressed in lymphomas and other malignancies, directly regulates the p53 tumor suppressor. However, recent evidence has suggested that Mdm2 also has p53- independent oncogenic activities, but the pathways that mediate these functions of Mdm2 are unresolved. Through biochemical purification, mass spectrometry, and subsequent experiments, we revealed a novel, specific association of Mdm2 with Nbs1, a component of the Mre11/Rad50/Nbs1 complex, which is required to maintain chromosomal integrity. Importantly, Mdm2 bound to Nbs1 independent of p53. Published and unpublished data show that reduced levels of Nbs1 or increased expression of Mdm2 results in defects in DNA integrity and increased rates of transformation that are, in part, independent of p53. Moreover, altered Mdm2 and Nbs1 expression are separately linked to human lymphoma development. Therefore, we hypothesize Mdm2 binds to Nbs1 and regulates its functions in a p53-independent manner, and that this interaction contributes to Mdm2-mediated transformation. We will test this hypothesis with in vitro, in vivo, and translational approaches. Experiments in Aim #1 will characterize the interaction between Mdm2 and Nbs1 and the consequences on Nbs1, the Mre11/Rad50/Nbs1 complex, and DNA damage signaling. Experiments in Aim #2 are designed to elucidate the effects on DNA integrity of Mdm2:Nbs1 interactions. Experiments in Aim #3 will evaluate the role Mdm2:Nbs1 binding has in transformation and in human lymphoma. Together these studies will reveal a novel function of Mdm2 in tumor development and provide much needed insight on the mechanisms involved in the p53-independent oncogenic pathways Mdm2 regulates. Results from these studies should ultimately lead to improved therapeutic strategies for the treatment of lymphomas and other human malignancies in which Mdm2 is overexpressed.